Staging: batman-adv: Count Ethernet header for incoming packets

The Ethernet header is counted when transmitting a packet, so it should also
be counted when receiving a packet. With this patch, the rx_bytes and tx_bytes
statistics behave like an ordinary Ethernet interface.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>